This invention relates generally to the processing of metals. More specifically, the invention is directed to processes and systems for the production or refining of specialty metals, such as titanium, aluminum, nickel, and their alloys.
Various metals and metal alloys are used to form relatively large structures which are used in demanding industrial applications. As an example, nickel-based superalloys are often used to form large turbine rotors and blades. Titanium alloys are frequently used for components in the cold section of a turbine, e.g., fan disks, fan blades, compressor discs, and compressor blades. They are also used to fabricate other types of high-value products. For example, a medical prosthesis can be cast or forged from a titanium material.
Specialty metals used for larger components are often produced as large bodies, using a sequence of processing techniques, such as vacuum induction melting, electroslag refining, and vacuum arc remelting. As an example, premium titanium alloys for aircraft engine applications are often formed by a process which includes cold-hearth melting of a titanium-based raw material, followed by one or more additional remelting steps, using vacuum arc remelting.
While the metal produced by these techniques is quite valuable, the processing through several steps can be expensive and time-consuming. For example, cold hearth melting systems usually employ a set of plasma torches or electron-beam guns to melt the raw metal feed material, and keep it in a molten state during processing. This type of equipment is very expensive to purchase, and requires a very high energy expenditure.
Considering their cost, the purity and quality of these specialty metals and metal alloys is often critical. Thus, special attention is paid to eliminating various impurities and foreign bodies from the metal product, e.g., an ingot. For example, hard alpha inclusions, which comprise titanium interstitially enriched with one or more of oxygen, nitrogen, or carbon, sometimes appear in titanium ingots. (Similarly, a variety of defects can sometimes appear in ingots formed of superalloys or other types of alloys). These defects, which are often introduced during the primary forming processes, can serve as initiation sites for points of weakness and potential failure of articles formed from the ingot.
The elimination or minimization of such defects remains a significant challenge to manufacturers, processors, and users. For example, nitrogen-rich inclusions generally form during the manufacture of virgin titanium, such as titanium sponge. Once formed, they must be removed or minimized during subsequent processing, sorting, or remelting operations. Fragmenting of the sponge into very small pieces may ameliorate the problem somewhat. The use of melting techniques which increase treatment time in the liquid state are also helpful. Still, efforts to-date have not always eliminated the presence of these defects.
In the case of titanium alloys, nitrogen-rich inclusions melt in a range above the melting range of titanium metal itself. The density of the inclusions is greater than the density of titanium metal and titanium alloys. Thus, removal of the inclusions by melting or floating is not practical. Dissolving of the nitrogen-rich inclusions in liquid titanium is very slow, but is currently the only practical solution. Hearth melt processing must be run very carefully and slowly to allow for density separation of the inclusions into the skull, or for redistribution of the interstitial element concentration by dissolution.
Another major source of defects in titanium-based articles is the presence of inclusions or contaminants of high-density or titanium-insoluble species. These contaminants are often tungsten-containing or other refractory compositions picked up during the recycle, recovery, and processing of titanium and titanium alloy scrap, also referred to herein as “revert.” For example, the undesirable materials are sometimes introduced during cutting processes which use torches or other cutting tools, and can be in the form of drill bits, saw blade teeth, cutting torch electrodes, and the like. The problem of removing these types of contaminants is especially difficult. Since they often have melting points well above that of titanium, heat treatments may be ineffective or impractical. Instead, they may have to be removed by other techniques, such as electron beam cold hearth remelting.
When metals like titanium are refined in an electroslag process, the product is very accessible to contaminants like those described above. For example, the inclusions and contaminants can fall off a melting feed-ingot and pass through an underlying layer of slag. They then become readily incorporated into the ingot product being formed in a mold below the slag.
As mentioned above, this invention also relates to the production of specialty metals. Titanium is a good example, since it is often used in alloys intended for critical applications. Commercial methods for obtaining titanium from various ores are well-known. Examples include chlorination of titanium ore to produce titanium tetrachloride, followed by reduction of titanium tetrachloride with sodium (the Hunter process), or with magnesium (the Kroll process).
Metals like titanium can also be produced in an electrolytic process, as described in U.S. Pat. No. 6,074,545 (Ginatta). In such a process, a vertically-disposed copper cylinder functions as the crucible, and a liquid metal pool serves as the cathode. The crucible is usually water-cooled, and also includes a base plate which is connected to a power supply. A graphite anode is positioned within the interior of the cylinder. The anode is also connected to the power supply, through a bus bar. The crucible contains molten-salt electrolyte (calcium or calcium compounds), which is analogous to the slag employed in an electroslag refining process. The electrolyte is maintained in a molten state by resistance-heating with electricity, originating at the power supply.
The compounds which contain the metal to be extracted are directed into the electrolyte by a conventional feeding mechanism. The feed compounds may be in either solid, liquid, or gaseous form. (In the case of titanium alloys, examples of the compounds are TiCl4, TiF3, TiBr4, AlCl3, VCl4, VCl3, VCl2, and the like.). As the feed compounds are reduced by electrolysis, gaseous byproducts are removed through a duct inside the anode. The product, e.g., titanium metal, is collected as a molten liquid at the cathode. A pool of the liquid is allowed to cool and solidify as an ingot within the cathode-mold. The ingot can be withdrawn by lowering a retractable base plate.
Electrolytic processes like those described in Ginatta may be very useful for producing titanium and other specialty metals. The Ginatta process, in particular, appears to be potentially capable of directly producing premium-grade titanium directly from raw materials like TiCl4. This technique can be a considerable processing advantage, in that it may allow one to bypass other complex steps related to titanium sponge formation and reconsolidation.
However, such processes still exhibit some of the serious drawbacks described previously. For example, the vertical alignment of Ginatta's melting and recrystallization stations would still permit inclusions and other impurities to pass directly from the anode chamber into the body of liquid metal which cools to become the product. This problem is especially acute when the raw material includes metal revert and other solid materials. Additional, time- and energy-consuming steps like remelting may therefore be required to eliminate inclusions and foreign bodies, as mentioned above.
With these concerns in mind, improvements in the way that titanium and other metals are produced and refined would be welcome in the art. The new techniques should be capable of efficiently reducing or eliminating inclusions and other impurities from the metal being refined or produced. For example, the removal of the foreign bodies should take place without the need for multiple re-melting stations, or other processing steps. Furthermore, the new processes should minimize the need for expensive heating equipment, like plasma torches or electron-beam guns. Moreover, the processes should be amenable to control mechanisms which can monitor and adjust critical parameters like melting temperature and electrical impedance. Finally, the processes should be compatible with other steps typically involved in metal production and refining, e.g., raw material processing or post-production stages like casting and forging.